Technical Field
The present invention relates to a particle detection device for measuring properties such as the number, size, or mass concentration of particles contained in the atmosphere or in the air in a cleanroom, for example.
Background Art
In one well-known class of devices for detecting particles suspended in a gas, sample air that contains the particles is input to the detection device and irradiated with laser light, and then properties such as the number, size, and mass concentration of the particles are measured by detecting the scattered light or incandescent light produced when the particles cross a region that is irradiated with the laser light.
Exhaust gas from diesel engines and exhaust gases produced from burning fuels that are composed primarily of carbon (such as coal, firewood, or biomass fuels, as well as gas produced by forest fires) contains primarily black carbon. When black carbon is momentarily heated by irradiating it with strong laser light such as that in a laser cavity or that from a pulse laser, the black carbon emits incandescent light due to the resulting black-body radiation. Detecting this incandescent light makes it possible to measure the number and size of black carbon particles. This method of detecting the incandescent light produced by black carbon is known as the laser-induced incandescence (LII) method (see Patent Document 1).
FIG. 10 is a block diagram of a signal processor in a conventional particle detection device. As illustrated in FIG. 11, scattered light and incandescent light signals received by a scattered light detector 129 and an incandescent light detector 130 are pulse waves, for example.
The threshold value comparison circuit 133 illustrated in FIG. 10 sets an appropriate threshold value for the received signals, which is used to determine which pulse waves to record. Next, the pulse waves to record are converted from analog values to digital values by AD converters 131 and 132. The digital pulse waves are then input to and recorded on a personal computer (PC) 134 or the like.
However, recording the pulse waves as-is as described above produces an extremely large amount of data, which results in longer signal processing times and a high load on the signal processor. A method such as the following offers a simpler alternative.
FIG. 12 is a block diagram of a signal processor for calculating particle size in a conventional particle detection device. In FIG. 12, components with the same reference characters as components in FIG. 10 are the same components as in FIG. 10. As illustrated in FIG. 12, the peak values of the received pulse waves are held by peak hold circuits 141 and 142. Then, the stored peak values are compared to a threshold value set in a threshold value comparison circuit 145, and the stored peak values that are larger than the threshold value are converted from analog values to digital values by the AD converters 143 and 144.
Here, assume that the peak values to compare are from the scattered light signals. There are two reasons for making this assumption. First, in most cases the particles will always produce scattered light but may not necessarily produce incandescent light. Second, if the scattered light and the incandescent light signals are both used for comparison purposes, then when the particle concentration increases, the amount of time occupied by the AD conversion process while getting the signals increases, which increases the amount of time during which particles cannot be detected (dead time).
Next, the digital scattered light and incandescent light signals are input to a CPU 147, and reset circuits 149 and 150 send reset signals to the respective peak hold circuits 141 and 142. Then the CPU 147 takes the input digital signals and converts the scattered light signals to particle size and the incandescent light signals to black carbon particle size according to peak value-particle size relationships configured in advance in a particle size setting circuit 146. Finally, the calculated particle size values are displayed on a display device 148.
The method described above makes it possible to get just the particle sizes (a small amount of data) from the large amount of data constituted by the original pulse waves, thereby making it possible to shorten processing time and reduce the load on the signal processor.